Barrier coatings are used in paper and paperboard packaging to provide barrier properties to paper and paperboard by reducing or eliminating the permeability of gases through the material and/or the absorption of liquids in the fiber structure.
Barrier coatings are required to prevent the egress from the package of flavors, aromas and other ingredients of the packaged product as well as to prevent the ingress into the package of oxygen, moisture, grease and other contaminants that might deteriorate the quality of the packaged product. Oxygen and water vapour are the gases for which barriers are normally tested but the barriers are useful for other gases as well, including carbon dioxide.
Various coatings have been applied to paper or paperboard substrates to provide composite materials that may be used for various purposes. Polymer dispersions or latexes have become attractive in recent years as a replacement for petroleum-based plastics for use as barrier materials. Barrier dispersions can be applied using conventional coating techniques, both online and off-line. Common applications of dispersion coatings are corrugated board, sacks, disposables, frozen and chilled food cartons, ream wrappings for copy paper, electronic packages and wallpaper base. The most commonly used latexes consist of polymers or copolymers of styrene, butadiene, acrylates, vinyl acetate and polyolefins dispersed in water. Several additives are used to reach the desired level of consistency, durability and runnability, e.g. colloidal stabilizers, thickeners, waxes, antifoaming agents, biocides and pesticides.
There is a need for biopolymer based barrier-coating compositions which are easy and inexpensive to produce, which have good barrier properties with respect to moisture, gas and grease and which have low brittleness. There is also a need for barrier coating compositions which can easily be separated from the cellulose fibres in recycling and repulping processes.
Natural polymers or biopolymers that come from renewable sources show many interesting properties in terms of film forming ability and resistance to oxygen and grease. However, the moisture sensitivity of biopolymers makes them inappropriate as barrier films for food packaging applications.
Another disadvantage of barriers based on natural polymers is the brittleness of the coatings, i.e. the sensitivity of barrier properties to mechanical stress applied in converting operations. Cracking of the barrier film causes the barrier properties to be lost.
The patent document WO 00/40404 describes coated films with improved barrier properties and relates to coating compositions which use a polymeric binder and a nano-scale particle size additive to provide improved moisture barriers. The area concerned is thermoplastic films and the coating compositions are suited for application to polypropylene and polyethylene films in order to improve the barrier characteristics of said polypropylene and polyethylene films and thereby making them acceptable for food packaging applications. The polymers used are not biopolymers and the intention is not to replace petroleum-based plastic films with more environmentally friendly coatings that provide sufficient barrier protection to paper or paperboard.
The patent document EP 1 736 504 describes improvement of barrier properties of a water soluble gas barrier material by adding nanoparticles of calcium carbonate. The polymers used are synthetic polymers and not biopolymers and the purpose is to improve oxygen barrier properties, not water vapour barrier properties.
The patent document WO 03/078734 describes a composition for surface treatment of paper by use of nanoparticles of synthetic layered silicates or precipitated calcium carbonate in a carrier fraction comprising plate-like pigment particles (talc and/or kaolin) and a binder such as a polymer latex (styrene-butadiene). The purpose is to improve the printing properties of paper, not to provide paper with improved barrier properties. Starch is mentioned as a surface sizing agent in order to improve the strength of the paper surface.
A study by Thang, X, Alavi, S and Herald, T, (Carbohydrate Polymers 74 (2008) 552-558 [available online 22 Apr. 2008]) considers corn starch with glycerol (0-20 wt %), urea (15 wt %) or formamide (15 wt %) as plasticizer. Montmorillonite clay is added to 6 wt %. Both the plasticizer and especially the clay concentrations are lower than in our ‘most preferred’ formulation. Furthermore, the materials were mixed by a twin-screw extruder, followed by grinding and dispersion of the ground material in water. Finally, the water dispersion was cast to self-supporting films. The WVTR was measured at 25° C. and 75% RH.
A study by Kampeerapappum, P, Aht-ong, D, Pentrakoon, D and Srikulkit, K, (Carbohydrate Polymers 67 (2007) 155-163 [available online 23 Jun. 2006]) refers to cassava starch in combination with chitosan (0-15% of the dry amount of starch). Chitosan is used as a compatibilizing agent to get a homogeneous dispersion of montmorillonite clay in the starch matrix. Clay was added at a concentration of 0-15 wt % of the dry amount of starch. Glycerol was used as a plasticizer. Self-supporting films with a thickness of about 70 μm were cast from the aqueous dispersion. WVTR was measured at 38° C. and 90% RH and values of 1000-2000 g/m2·d were reported. These values are 10-20 times higher than for paper coatings, measured under the same conditions (Example 6 below).
In a study by Cyras, V P, Manfredi, L B, Ton-That, M-T and Vázquez, A, (Carbohydrate Polymers 73 (2008) 55-63 [available online 22 Nov. 2007]) native starch (not chemically modified) is used in combination with 0-5 wt % Na-Cloisite. Self-supporting films were cast from water solution and the resulting film thickness was 250 μm. The equilibrium water uptake and water absorption rate was measured, but the article do not report any measurement of water vapour barrier properties.